You can add through the ingredients tab. Once you have that open, click on water in the drop down, there should be an icon at the top left that says Add Water. Click that icon, and input your mineral levels. This can be done, but I usually just make a note in notes that I was using RO water, and I always use RO now so have even stopped doing that. I adjust my water through Bru'NWater though, if you want to use Beersmith's water calcs, you will have to add the mineral content on that calculation, but still should not need to have RO in the list of available water, unless I am missing something